Field of the Invention
The field of the invention is that of electromagnetic wave detectors made with III-V semiconductor materials so as to define quantum well structures.
The working of such detectors is based on the occurrence of electronic transitions between permitted energy levels (e1 and e2) within the conduction bands of semiconductor quantum structures. FIG. 1a gives an example of this type of transition in a well having two discrete permitted energy levels for the electrons. By applying an electrical field to this type of configuration, it is possible to extract electrons from the well in giving preference to the electrons located at the second quantum level. Thus, through the collection, in the external electrical circuit, of these electrons coming from the second quantum level to which they have been carried by an illumination hv, it is possible to detect this illumination.
To achieve high absorption of the illumination to be detected, it is possible to use a large number of wells within detectors based on this quantum principle. FIG. 1b shows a multiple-well configuration of this kind.
The problem encountered with the prior art structures, described here above, lies in the high rate of carrier recombination. This is due especially to a barrier layer between successive wells. This barrier layer has a small thickness which is close to that of the quantum wells.
Photovoltaic variants of these detectors have been proposed in the literature [see Borge VINTER, xe2x80x9cDetectivity of a three-level quantum well detectorxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 30, p. 115 (1994)].
The problem encountered with prior art structures, as described here above, lies in the high rate of carrier recombination.
This recombination restricts the performance characteristics of these detectors and especially their operating temperature.
In the case of the photovoltaic device, this limitation is due to an excessively thin barrier layer between the two neighboring wells constituting the photovoltaic structure.
To place substantial limits on the recombination rate of the carriers, the invention proposes the introduction, in the detector, of a storage layer different from the absorbent layer (quantum well), and to do so by means of a transfer barrier with a great width as compared with that of the quantum well. By thus separating the absorption function (in the quantum well) and the photocarrier read function (in a storage layer), the performance characteristics of the detectors are improved through prevention of the recombinations of carriers.
To enable the flow of the photo-excited electrons in a storage layer, the transfer barrier has a conduction potential profile that shows a decrease starting with the quantum well.
More specifically, an object of the invention is an electromagnetic wave detector comprising a stack of layers made of III-V semiconductor materials, the conduction band profile of said materials defining at least one quantum well, said quantum well having at least one first discrete energy level populated with electrons that are capable of passing to a second energy level under the absorption of an electromagnetic wave and means for the reading of said electrons in the second energy level wherein the stack of layers of semiconductor materials furthermore comprises an electron storage layer separated from the quantum well by a transfer barrier layer, the thickness of the transfer barrier layer being about one order of magnitude greater than the thickness of the quantum well, the lower energy level of the conduction band of the transfer barrier layer being greater than those of the quantum well and the electron storage layer and decreasing from the quantum well to the electron storage layer so as to further the flow of electrons from the second energy state to the electron storage layer.
Thus, the detector of the invention comprises:
a quantum well having an intersubband absorption at the desired energy, this layer being quite similar to the quantum wells commonly used in the quantum well detectors [B. LEVINE, xe2x80x9cQuantum well infrared photodetectorsxe2x80x9d, Journal of Applied Physics, Volume 74, No. 8, R1. (1993)];
a transfer barrier that behaves like a loss of potential in which the photo-excited electrons may be transferred;
a layer for the storage of the photo-excited electrons;
means for reading the photosignal.
According to a first variant of the invention, the transfer barrier may consist of a semiconductor alloy whose composition varies along the thickness of said barrier so that the conduction potential decreases with distance from the well.
According to a second variant of the invention, the transfer barrier may be made out of piezoelectric material that generates a natural electrical field, enabling the conduction potential of the transfer barrier to be given the required profile.
According to a third variant of the invention, the semiconductor structure may also be placed directly under an electrical field to obtain the desired conduction potential profile for the transfer barrier.
Furthermore, the reading of the photodetection signal may be done differently.
It may relate, for example, to a measurement of parallel photocurrent using ohmic contacts that contact the storage layer without contacting the absorbent quantum well.
It may also be a photovoltaic reading of the voltage due to the spacing between the electrons in the storage layer and the layer of the absorbent well.